Half of the UK's top ten best-selling books on Amazon are currently colouring-in books for adults, highlighting a new craze that has got grown-ups reaching for their felt-tips.
Leading the way is a book called Secret Garden, illustrated by Aberdeenshire artist Johanna Basford, which has sold more than 1.4m copies and is sitting at the top of Amazon's bestseller list in the UK and the US.
Basford thinks part of the appeal is switching off from the digital world. "You're not plugged in. You're not looking at emails or updates. You're not looking at Twitter. It's not a competitive sport. It's just something you can do quietly by yourself and be creative," she tells The Herald.
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The Times says colouring-in books are enjoying a "remarkable renaissance" thanks to an unlikely new fan-base: stressed-out adults.
The craze appears to have started in France, where publishers have been marketing the books as an exercise to combat anxiety. "The phenomenon is spreading: in Australia adults gather for 'colouring circles' in coffee shops, where they sit in silence, fastidiously staying inside the lines," says The Times.
In the UK, Waterstones has recorded a 300 per cent rise in year-on-year sales to December 2014.
"What on earth is going on here?" asks Harry de Quetteville at the Daily Telegraph. "Have hundreds of thousands of people lost their minds? Has the creeping infantilisation of the adult world reached a new nadir?"
He suggests that the "madness" started with JK Rowling, when "grown men and women felt that it was socially acceptable to read, in public, children's books about boy wizards flying around on broomsticks".
At least Harry Potter books had words in them, he adds. "There was a certain threshold that adults had to pass. They had to be able to read. This new picture fad is worse."
But The Guardian's Philippa Perry praises the new trend, warning that adults are in danger of forgetting how to play.
"Colouring in is not a passive act: you need to make creative decisions about which colours to choose and, while you concentrate on not going over the lines, other parts of your mind may be freed up in ways that allow you to become more creative," she says.
"Letting the mind wander from whatever it is you are colouring in is a form of play. Young children often learn best when they are playing, so why shouldn't we apply that principle to adults too?"
